In a communications system, a transmitter sends data to a receiver through a channel. In the case of a wireless channel, the transmitted waveforms suffer from multipath fading due to reflection, refraction, and diffraction, which ultimately results in inter-symbol-interference between transmitted symbols of the signal. This is particularly problematic for modern broadband wireless communications systems, e.g., OFDM systems, which offer high data rate services. Particularly for such high data rate systems, multipath fading is especially difficult to mitigate.
Many current communications systems mitigate ISI by using a cyclic prefix (“CP”) for each transmitted symbol. The CP is a copy of the latter portion of a transmitted symbol that is prepended to the transmitted symbol. The CP acts as a buffer region where delayed information for the previous symbol can be stored by the receiver. The receiver has to exclude all the samples from the CP since those samples can be corrupted by the previous symbol. Furthermore, the CP interval length can vary to accommodate different multipath environments. Typically, the CP interval length is determined by the expected duration of the multipath channel in the operating environment. As such, a DVB-T system has been configured to have four different CP intervals, including ¼, ⅛, 1/16 and 1/32. However, the multipath delay can be longer than these set intervals causing unreliable decoding of the signal.
FIG. 1 illustrates symbols of an OFDM signal, where each of the symbols has a cyclic prefix. Symbols m−1, m, and m+1 for a single carrier of the OFDM signal can be transmitted sequentially in the time domain. The symbol m comprises a cyclic prefix 10 having NCP points and a body 12 having N points. The CP 10 is discarded to avoid any ISI to the symbol m from the previous symbol m−1. However, when a multipath delay spread for the signal is greater than the CP 10 length, the ISI will affect the body 12 of the symbol m, causing the data in the symbol m to be unreliable.
Therefore, it is desirable to provide new methods and systems for processing a signal to improve the reception of the signal even when the multipath delay spread is greater than the length of a CP for the signal.